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Chromosomal translocations bring the previously unlinked segments of the genome together by virtue of the exchange of parts between non-homologous chromosomes. Although some translocations are not associated with a new phenotype, others may result in disease due to the modulation of protein expression or the synthesis of a new fusion protein.
There are two main types of chromosomal translocations which occur, these being reciprocal translocations (also known as non-Robertsonian) and Robertsonian translocations. Further, translocations can be balanced (in an even exchange of material with no genetic information extra or missing) or unbalanced (where the exchange of chromosome material is unequal resulting in extra or missing genes).
Reciprocal (non-Robertsonian) translocations usually result in an exchange of material between non-homologous chromosomes and are found in about 1 in 600 newborns. Such translocations are usually harmless and may be found through prenatal diagnosis. However, carriers of balanced reciprocal translocations exhibit an increased risk of creating gametes with unbalanced chromosome translocations thereby leading to miscarriages or children with abnormalities.
Robertsonian translocations involve two acrocentric chromosomes that fuse near the centromere region with loss of the short arms. The resulting karyotype has only 45 chromosomes since two chromosomes have fused together. Robertsonian translocations have been observed involving all combinations of acrocentric chromosomes. The most common translocation involves chromosomes 13 and 14 and is seen in about 1 in 1300 persons. Like other translocations, carriers of Robertsonian translocations are phenotypically normal, but exhibit a risk of unbalanced gametes which lead to miscarriages or abnormal offspring. For example, carriers of Robertsonian translocations involving chromosome 21 exhibit a higher probability of having a child with Down syndrome.
Diseases which may result from the occurrence of a translocation include:                (i) Cancer—several forms of cancer are caused by translocations; this mainly having been described in leukemia (eg. acute myelogenous leukemia and chronic myelogenous leukemia).        (ii) Infertility—this can occur where one of the would-be parents carries a balanced translocation, where the parent is asymptomatic but conceived foetuses are not viable.        (iii) Down syndrome—in some cases this is caused by a Robertsonian translocation of about a third of chromosome 21 onto chromosome 14.        
Specific examples of chromosomal translocations and the disease with which they are associated include:                t(2;5)(p23;q35)—anaplastic large cell lymphoma        t(8;14)—Burkitt's lymphoma (c-myc)        t(9;22)(q34;q11)—Philadelphia chromosome, CML, ALL        t(11;14)—Mantle cell lymphoma (Bcl-1)        t(11;22)(q24;q11.2-12)—Ewing's sarcoma        t(14;18)(q32;q21)—follicular lymphoma (Bcl-2)        t(17;22)—dermatofibrosarcoma protuberans        t(15;17)—acute promyelocytic leukemia (pml and retinoic acid receptor genes)        t(1;12)(q21;p13)—acute myelogenous leukemia        t(9;12)(p24;p13)—CML, ALL (TEL-JAK2)        t(X;18)(p11.2;q11.2)—Synovial sarcoma        t(1;11)(q42.1;q14.3)—Schizophrenia        t(1;19)—acute pre-B cell leukemia (PBX-1 and E2A genes).        
The shorthand t(A;B)(p1;q2) is used to denote a translocation between chromosome A and chromosome B. The information in the second set of parentheses, when given, gives a precise location within the chromosome for chromosomes A and B respectively—with p indicating the short arm of the chromosome, q indicating the long arm, and the numbers of p and q refers to regions, bands and sub-bands seen when staining the chromosomes under microscope.
As detailed above, chronic myelogenous leukemia is an example of a neoplastic condition which is caused by a chromosomal translocation. However, unlike many neoplastic conditions, its treatment prospects are quite good if it can be effectively diagnosed and monitored.
In virtually all cases of chronic myelogenous leukemia, a specific translocation is seen. This translocation involves the reciprocal fusion of small pieces from the long arms of chromosome 9 and 22. The altered chromosome 22 is known as the Philadelphia chromosome (abbreviated as Ph1). When the breakpoint of the Ph1 chromosome was sequenced, it was found that the translocation creates a fusion gene by bringing together sequences from the c-ABL proto-oncogene and another BCR (breakpoint cluster region). The BCR-ABL fusion gene encodes a phosphoprotein (p210) that functions as a dysregulated protein tyrosine kinase and predisposes the cell to become neoplastic. This hypothesis is supported by finding that expression of p210 results in transformation of a variety of hematopoietic cell lines in vitro and that mice transgenic for the human BCR-ABL gene develop a number of hematologic malignancies.
Another well studied example of a translocation generating cancer is seen in Burkitt's lymphoma. In some cases of this B cell tumor, a translocation is seen involving chromosome 8 and one of three other chromosomes (2, 14 or 22). In these cases, a fusion protein is not produced. Rather, the c-myc proto-oncogene on chromosome 8 is brought under transcriptional control of an immunoglobulin gene promoter. In B cells, immunoglobulin promoters are transcriptionally quite active, resulting in over expression of c-myc, which is known from several other systems to exhibit monogenic properties. Accordingly, this translocation results in aberrant high expression of an oncogenic protein.
The classical method of diagnosing chromosomal translocations, such as those observed in chronic myelogenic leukemia, is by karyotyping. For many translocations, however, it is now possible to detect the translocation by PCR, using primers which span the breakpoint. In some cases, the PCR technique can also be used for sensitive detection and monitoring of treatment. Monitoring to determine the effect of treatment has become increasingly important for diseases such as chronic myeloid leukemia and acute promyelocytic leukemia as increasingly effective treatment has been developed. For monitoring in these 2 diseases, the starting material for the PCR is RNA. The translocation breakpoint is within the introns of the respective genes and, as a consequence, RNA splicing removes the sequence of RNA transcribed by introns and results in only one or a very limited number of final mRNA products being produced, despite the very large number of different translocations which are present in the patient population.
However, the use of RNA as the starting material to detect and quantify the translocation by PCR suffers the disadvantage that RNA is a difficult molecule to work with due to its inherent susceptibility to degradation. DNA is a more stable molecule. However, the initial identification and characterisation of the breakpoint in the context of DNA is much more difficult since cluster regions of chromosomal fusion sites often span large introns of several tens of thousands of nucleotides. These sizes are too large for direct coverage by a single PCR reaction. There therefore exists an ongoing need to develop means for routinely conducting breakpoint analyses on DNA samples.
In work leading up to the present invention, a novel multiplex amplification reaction has been developed which enables the localisation and analysis of a breakpoint in a DNA sample. Despite the precise position of the breakpoint being unknown, the method of the present invention nevertheless enables diagnosis of the existence of the breakpoint in a DNA sample and the isolation and analysis of the breakpoint region using a relatively modest and simple multiplex amplification reaction. The design of this amplification reaction results in the advantage that generation of long PCR products is not required. Still further, the optional incorporation of a primer hybridisation tag region at the 5′ end of the amplification primers enables the rapid generation of large copy numbers of the amplicons generated using these primers and therefore facilitates the isolation and analysis of the amplicons.